1. Field of the Invention
The present invention relates to a polypropylene resin composition from which a sorbitol derivative added thereto is not extracted or does not escape from the resin composition in hot water and which is excellent in transparency and more specifically to a polypropylene resin composition which can provide molded products excellent in transparency through blow molding.
2. Description of the Related Art
Polypropylene resins have widely been used in various fields, for instance, in foods (for instance, as materials for the production of packages for foods; containers for frozen dessert, for fast foods and for margarine; and trays); and in medical machinery and tools (for instance, as materials for the production of syringes for injectors, spitz tubes, containers for infusion solutions and cylinders) since they are excellent in mechanical properties, heat resistance and resistance to chemicals.
However, they do not exhibit satisfactory transparency. It is very important for materials used in such fields as containers for foods and medical machinery and tools that the presence or absence of impurities or foreign substances such as refuse or the like in the contents of a container can be externally confirmed and it is not desirable that the color tint of the contents be changed when it is observed through the container. For this reason, it is desired to use a polypropylene resin excellent in transparency, in these fields.
There has widely been used a method for improving the transparency of polypropylene resins which comprises incorporating a sorbitol derivative into the resins.
However, when a polypropylene resin composition obtained by incorporating such a sorbitol derivative into a polypropylene resin is immersed in hot water for the purpose of sterilization or the like, the sorbitol derivative is extracted or escapes from the resin composition and is dissolved in the hot water and after the hot water is cooled, the derivative forms suspended matter in the water. This necessarily becomes a cause of problems of safety and hygiene.
Examples of sorbitol derivatives which can improve the transparency of polypropylene resins include 1.3, 2.4-di-benzylidene sorbitol, 1.3, 2.4-di-(methylbenzylidene)sorbitol and 1.3, 2.4-di-(ethylbenzylidene)sorbitol.
Polypropylene resin compositions to which 1.3, 2.4-di-(methylbenzylidene) sorbitol or 1.3, 2.4 - di(ethylbenzylidene)sorbitol is added among others are excellent in transparency, but generate a large quantity of suspended matter when they are immersed in hot water and, therefore, a problem of safety and hygiene arises.
On the other hand, when polypropylene resin compositions to which 1.3, 2.4-di-benzylidene sorbitol is added are immersed in hot water, suspended matter is formed in a relatively small amount in the hot water, but the problem of safety and hygiene is not yet solved completely and the transparency thereof is still insufficient. Thus, any compositions which simultaneously satisfy the foregoing two requirements, i.e., compositions excellent in transparency and free of suspended matter when they are immersed in hot water have not yet been proposed.
Moreover, some polypropylene compositions improved in transparency by the addition of sorbitol derivatives have been proposed, but if they are subjected to blow molding to form molded products, the transparency of the products is impaired.